Which one is correct?
"I don't know what did he ask about".
"I don't know what he asked about".
e2e4, with respect, you posted a contradiction to a member with 10,000-plus posts, a member who is a native English speaker and an English teacher.
If your post had been within minutes of his, and perhaps you hadn't seen it when you posted, I would have understood. But three hours later, you come along and say he was wrong.
Does that seem like a logical approach to you?
Do the native speakers make mistakes? Sure we do. But if you thought his answer was wrong, there are other ways to go about it.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.